A. Field of the Invention
The present invention is related to methods for manufacturing coils and more particularly to a method for manufacturing coils in which a layer of paper covered with a thermo curable resin is applied between conductors and uniformly thermoset and thermo-cured with the resultant heat produced by applying a constant density electric current to all the conductor portions of the coil—Joule effect—while an axial pressure is applied to it.
B. Description of Related Art
In processes for manufacturing coils, there are usually applied methods of compacting conductors and methods for increasing the rigidity of the coil in order to increase its tolerance to short-circuit mechanical forces and thus avoiding problems related to impedance variations of the final product.
The method of compacting conductors by any means, allows the manufacturing of smaller core-coil units, and the use of lower cost smaller oil containers.
In order to help to maintain the integrity and rigidity of the coil and additionally to electrically isolate each conductor layer, the conductors are impregnated with a non-conductive resin before compacting the coil, and subsequently the resin is heat cured inside an oven.
A method for manufacturing coils is claimed in the Japanese patent No. JP2-040902 of Ogawa et al. wherein an impregnated resin is thermoset by Joule heat while being pressurized in the coil axial direction.
In Ogawa's patent, it is disclosed that the Joule effect only produces a partial heating of the coil periphery in order to thermoset the resin, and uses a contraction magnetic force produced by the electric current through the conductors for compressing the coil in an axial direction.
Since Ogawa's method only achieves the heating of the periphery of the coil, the resin impregnated in the conductors near the core of the coil is not uniformly thermoset and thermo cured.
In order to achieve an uniform heating of the entire coil and thus a complete an uniform thermo curing of all the resin impregnated in the coil, applicant developed a method for manufacturing coils which achieves an uniform and complete curing of the paper impregnated with epoxy thermo curable resin placed between conductors by applying an electric current uniformly to all portions of the coil thus completely heating the coil, including the conductors near the core.
Applicant's method comprise: connecting both bobbin terminals to a variable electric source; calculating an initial amperage to be applied to the primary and secondary bobbin windings; activating the variable electric source; detecting the temperature obtained in the both windings; if the temperature is equal to the temperature defined by the curing graphic over current time, then the amperage must be maintained in order to maintain the windings temperature equal to the value predefined in the curing graphic; if the temperature is greater than the temperature defined by the curing graphic over current time, then the amperage must be decreased until the predefined temperature is achieved; if the temperature is lesser than the temperature defined by the curing graphic over current time, then the amperage must be increased until the predefined temperature is achieved; repeat the temperature detecting step until the last time value of the curing graphic is achieved; and deactivating the variable electric source until the last time value of the curing graphic is achieved.
By the method of the present invention, it is achieved the heating of all coil layers at a relatively uniform temperature thus curing the epoxy resin impregnated in all the papers placed between all conductors.
By applying adequate amperage to the coil it is achieved a reduction of time of the thermo curing cycle in comparison with conventional convection heating methods using ovens.
The method of the present invention uses the transformer effect of the coil for heating simultaneously all portions of the coil without needing ferromagnetic cores, or coils magnetically coupled with air core having different tensions.
Thanks to an optimum heat distribution in the coil achieved by the method of the present invention it is obtained an energy saving compared with conventional heating methods.